Go: What is the difference between slice and array?

Array — fixed-length, value-type container with length baked into the type. Slice — a lightweight, dynamic view over a contiguous block of memory.

Arrays are assigned by value — the entire array is copied. This can be expensive for large arrays but makes them safe to use without worrying about shared state.

1arr := [4]string{"a", "b", "c", "d"}
2copy := arr
3copy[0] = "z"
4fmt.Println(arr[0])  // "a" — original unchanged

Slices are three words in memory: a pointer, length, and capacity. When you slice an existing array or slice, the new slice shares the backing memory. This makes slicing efficient but means mutations can affect the original.

1original := []string{"a", "b", "c", "d"}
2view := original[1:3]   // ["b", "c"] — shares memory
3view[0] = "z"
4fmt.Println(original[1])  // "z" — original changed!
5
6// append may allocate new backing array
7view = append(view, "x")  // may or may not affect original

When to use:

• Array — small fixed buffers, map keys (arrays are comparable), protocol constants.
• Slice — almost everything else: function parameters, return values, dynamic data.

Common mistakes:

• Passing large arrays by value (copies everything).
• Not capturing the return of append — append(s, elem) returns a new slice header.
• Expecting copy() to deep-copy slices — it copies elements but shares the underlying array.
• Using make([]int, 10) expecting empty slice with capacity — it has len=10.